1. Field of the Invention:
This invention relates to the manufacture of synthetic fuels and is particularly concerned with a method for the production of methane from synthesis gas and similar mixtures of carbon monoxide or carbon dioxide and hydrogen.
2. Description of the Prior Art:
Conventional processes for the manufacture of synthetic fuels by the gasification of coal or other carbonaceous solids generally require the reaction of steam with carbon at temperatures between about 1200.degree. and about 2500.degree. F. to produce a gas containing hydrogen and carbon monoxide and the subsequent reaction of a portion of the carbon monoxide with steam at lower temperatures to produce carbon dioxide and additional hydrogen by means of the water-gas shift reaction. Following this, the gas is usually treated to remove carbon dioxide and sulfur compounds and then fed to a catalytic methanation unit for reaction of the carbon monoxide and hydrogen to produce methane and water vapor.
The water-gas shift reaction is an important part of processes of the type referred to above because of the need for a high hydrogen concentration in the methanation step. This reaction is equilibrium limited and must be carried out at relatively low temperatures if the required yields of hydrogen are to be obtained. To facilitate this, catalysts such as copper on zinc oxide and ferric oxide promoted by chromic oxide have been used. More recently, it has been disclosed that alkali metal compounds are effective water-gas shift reaction catalysts at temperatures of from about 400.degree. to 700.degree. F. and are less affected by sulfur compounds than many of the earlier materials. It has also been shown that the alkali metal compounds will catalyze the steam-carbon reaction.
The reaction of carbon monoxide and hydrogen to produce methane is normally carried out in the presence of an iron, nickel or cobalt catalyst. The most effective catalysts have generally been compositions containing nickel as the primary constituent and including small amounts of a promoter such as thorium oxide, magnesium oxide, aluminum oxide, potassium oxide, calcium oxide, potassium carbonate, manganese or the like. The promoters alter the chemical and physical characteristics of the catalyst surface and in low concentrations tend to improve the yield or selectivity obtained with the catalyst. These materials may be employed in conjunction with carriers such as kieselguhr, pumice, infusorial earth, asbestos, silica, alumina or the like. Nickel or alumina containing a small amount of potassium carbonate as a promoter is currently considered by those skilled in the art as the best available methanation catalyst.
Although methanation catalysts of the type referred to above are reasonably effective, experience has shown that such materials are highly sensitive to sulfur compounds and are quickly poisoned. To avoid this, the gases fed to the methanation reactor must be treated to remove both organic and inorganic constituents containing sulfur. This is generally done by first scrubbing the gas stream with a solvent such as methanol to eliminate most of the hydrogen sulfide and mercaptans and then removing the last traces of these impurities by adsorption on reduced zinc oxide or a similar adsorbent. Periodic regeneration of the catalysts by treatment with hydrogen is generally necessary. These feed gas purification and catalyst regeneration steps are expensive.